Worldwide agriculture suffers, especially in developing countries such as in Africa, from annual huge losses of crop and other economic plants due to plant diseases. More than 30% of the food, fiber, feed and energy produced in crop production systems are destroyed by insects and diseases annually on a global scale. These yield losses are high as a result of low-input production systems due to the non-affordability of synthetic fungicides to farmers in developing countries that depend on non-conventional disease management practices often providing doubtful results.
In contrast, crop and plant producers in developed countries rely largely on synthetic pesticides to control plant diseases. It is an established fact that the use of synthetic chemical pesticides provides many benefits to crop producers. These benefits include higher crop yields, improved crop quality and increased food production for an ever increasing world population. The development of a wide range of chemicals with different formulations has enabled man to control a wide range of plant pathogens and substantially increased crop yields. More than a decade ago crop producers spent nearly $20 billion on pesticides and $150 million on other plant protection techniques, worldwide, to control pests in general. The world market share of fungicides alone was 20% in recent years whilst Europe accounted for 30% of the market. However, the same level of pathogen control has not been realized in developing countries, partly as a result of pesticide technology not being accessible to most resource poor farmers. Failure of modern approaches, technology and chemicals to reach farmers in developing countries is solely the result of high costs in relation to the value of the crops cultivated by these farmers. Consequently, crops are routinely subjected to attack from a wide spectrum of a diversity of pathogens and these farmers constantly experience serious crop damage. Moreover, yield losses are on the increase despite high pesticide usage, even in developed countries. Furthermore, control of plant diseases is not easily achieved with a single application of fungicide but requires frequent applications during the crop-growing period. However, synthetic pesticides may pose a couple of threats and hazards to the environment, especially when improperly used by farmers in developing countries who lack the technical skill of handling them, and who fail to adopt to this technology easily. This may result in undesirable residues left in food, water and the environment, and may cause toxicity to humans and animals, contamination of soils and groundwater and may lead to the development of crop pest populations that are resistant to treatment with agrochemicals. Especially sulfur and copper containing synthetic fungicides are toxic to mammals, wildlife and many beneficial insects.
Furthermore, in Africa and the Near East, obsolete pesticides have become a source of an additional great environmental concern. Some stocks are over 30 years old and are kept in poor conditions because of inadequate storage facilities and lack of staff trained in storage management. Obsolete pesticide stocks are potential time bombs. Leakage, seepage and various accidents related to pesticides are quite common and widespread.
Additionally, that frequent application of fungicides has resulted in fungal mutation and, subsequently new resistant strains (Khun, 1989, Pesticide Science 14:272-293), the combat of which usually requires stronger pesticides with again stronger impacts on the environment.
For all these reasons there is a considerable and increasing consumer resistance especially in the developed countries, initiated politically by the green parties, towards the use of synthetic chemicals/pesticides especially, supplying a rationale for a shift from chemical pesticides applications to the use of naturally derived plant protecting agents in order to reduce the pollution and health risk caused by pesticides.
As a result, research on the possible utilization of biological resources and its application potential in agriculture has become very relevant. A promising approach in this regard is the use of natural plant products as an interesting alternative to synthetic chemicals due to the apparent less negative impact on the environment.
This especially applies to the search for environmentally friendly bioactive naturally derived components and agents with, for example, broad-spectrum antimicrobial activity.
Natural products from plants are expected to have a narrow target range and highly-specific mode of action, to show limited field persistence, to have a shorter shelf life and present no residual threats. They are generally safer to humans and the environment than conventional synthetic chemical pesticides and can easily be adopted by farmers in developing countries who traditionally use plant extracts for the treatment of human diseases.
A further rationale for exploring the use of plant extracts or natural products as biological pesticides more extensively can be found in the plant itself. Plants have evolved highly specific chemical compounds that provide defense mechanisms against attack by disease causing organisms, including fungal attack, microbial invasion and viral infection (Cowan, 1999, Clinical Microbiology Reviews 12:564-582). These bioactive substances occur in plants as secondary metabolites, and have provided a rich source of biologically active compounds that may be used as novel crop-protecting agents. In nature some plants have the potential to survive very harsh environmental conditions. This has initiated the postulate that such plants might be utilized as sources for the development of natural products to be applied in agriculture by man as natural herbicides, bactericides, fungicides or products in crude or semi-purified form. Secondary plant metabolites are distinct from primary metabolites in that they are generally non-essential for the basic metabolic processes such as respiration and photosynthesis. They are numerous and widespread, especially in higher plants and often present in small quantities (1-5%) as compared to primary metabolites (carbohydrates, proteins, lipids). Secondary metabolites are probably produced when required in the plant system and are synthesized in specialized cell types. Ecologically, secondary metabolites play essential roles in attracting pollinators, as adaptations to environmental stresses and serve as chemical defenses against insects and higher predators, micro-organisms and even other plants (allelochemicals).
Abiotic stress such as nutrient limitation, light intensity, water stress and others has been considered to trigger the formation of secondary metabolites. A biotic stress related type of plant-pathogen interaction involves the production of metabolites as part of a plant defense arsenal against microbial invasion and is considered disease determinants. Secondary metabolites with anti-microbial properties include terpenoids (e.g. iridoids, sesquiterpenoids, saponins), nitrogen- and/or sulphur containing (e.g. alkaloids, amines, amides), aliphatics (especially long-chain alkanes and fatty acids) and aromatics (e.g. phenolics, flavonoids, bi-benzyls, xanthones and benzoquinones).
Another related area of organic fanning systems is the potential to apply natural plant extracts as either plant growth regulators or bio-stimulants. Many natural plant compounds have been identified that affect the growth and development of plants. Secondary metabolites from plants may show also bio-stimulatory activities in plants, other plants included. Probably the most effective compound to enhance crop yield, crop efficiency and seed vigour has been identified as a brassinosteroid (Mandava, 1988, Plant Physiology Plant Molecular Biology 39:23-52). Brassionosteroids have also been identified as bio-stimulatory substances from a plant extract mixture deriving from a specific Pink species and a specific Alfalfa species (EP 1 051 075 B1). An elevated interest therefore exists to identify natural plant compounds with the ability to manipulate plant growth and development over a short period, e.g. a growing season.
An additional consideration is that plants whose extracts, for example show antimicrobial and/or bio-stimulatory properties, could be cultivated as alternative agricultural crops for serving as sources of active compounds in the production of natural pesticides or plant growth regulators.
Although plants are a valuable source for the development of new natural products with the potential to be used for disease management in organic crop production systems only a small number of plants has been investigated for possible use in plant disease control in agriculture. However, related to this relatively small number of investigated plants a relatively large number of scientific research activities has been done during the last couple of years. Some of them are listed as follows:                It was shown (Pretorius et al., 2002, Annals of Applied Biology 141:117-124) that mycelial growth inhibition was obtained with extracts from two species of the subclass Liliidae, namely Aristea ecklonii and Agapanthus inapertus. The crude extract of A. ecklonii performed best of all extracts as it totally inhibited the mycelial growth of all seven of the plant pathogenic test organisms and outperformed the inhibition by a broad spectrum synthetic fungicide (carbendazim/difenoconazole). Crude extracts of A. inapertus showed complete inhibition of four and strong inhibition of the remaining three plant pathogenic fungi.        Plant seeds also contain compounds with antimicrobial properties. Seed extracts of 50 plant species, belonging to different families, were evaluated for their ability to inhibit the growth of Trichoderma viride in vitro (Bharathimatha et al., 2002, Acta Phytopathologica et Entomologica Hungarica 37:75-82). Of the various seed extracts, that of Harpullia cupanioides (Roxb.), belonging to the family Sapindaceae, displayed very high antifungal activity.        The natural plant product Milsana®, extracted from the giant knotweed (Reynoutria sacchalinensis), is probably best known (Daayf, 1995, Plant Disease 79:577-580). The product has been reported to control powdery mildew, caused by Sphaerotheca fuliginea, in long English cucumber under greenhouse conditions and also showed broad spectrum activity against powdery mildew of tomato, apple and begonia as well as downy mildew of grapevine and rust of bean.        Extracts from the leaves and seed kernels of the neem tree (Ume et al., 2001, The science and application of neem, meeting proceedings, Glasgow, U.K. April-2001. Pp. 33-37) were tested for antifungal activity against the plant pathogenic fungus Sclerotium rolfsii [Corticium rolfsii]. All the extracts showed some effect against different growth stages of the fungus, but the effects were fungistatic rather than fungitoxic.        Amadioha (2002, Archives of Phytopathology and Plant Protection 35:37-42) evaluated the antifungal activities of the different extracts of A. indica. The oil extract from seeds as well as water and ethanol leaf extracts of the plant were effective in reducing the radial growth of Cochliobolus miyabeanus in culture and in controlling the spread of brown spot disease in rice.        A study directed towards identifying bio-stimulatory properties in plant extracts was performed by Cruz et al. (2002, Acta Horticulturae 569:235-238) by treating the roots of bean, maize and tomato with an aqueous leachate of Callicarpa acuminate. The aqueous extract of C. acuminata inhibited the radicle growth of tomato but had no effect on root growth of maize or beans.        According to Singh et al. (2001, Journal of Crop Production 4:121), allelochemicals isolated from some plants show strong bio-herbicidal activity at high concentrations, but at low concentrations these extracts can promote crop seed germination and seedling growth, hence showing a potential to be applied as bio-stimulatory agents or growth promoting substances in agriculture.        Extracts from some lucerne cultivars had a stimulatory effect in terms of seed germination as well as root and hypocotyl growth, whereas others showed the direct opposite effect, confirming that crop plants can also be affected by plant extracts aimed at controlling weed growth (Tran and Tsuzuki, 2002 Journal of Agronomy and Crop Science 188:2-7).        Leksomboon et al. (2001, Kasetsart Journal, Natural Sciences 35:392-396) demonstrated the antibacterial effect of leaf and other aqueous extracts of Hibiscus sabdariffa, Psidium guajava, Punica granatum, Spondias pinnata and Tamarindus indica against Xanthomonas axonopodis, the casual agent of citrus canker under both laboratory and field conditions.        Another natural product, carvone, derived from dill and caraway seed, has been developed to inhibit the growth of storage pathogens and to suppress sprouting of potatoes in the warehouse (Moezelaar et al., 1999, In: Modern fungicides and antifungal compounds II; Intercept Limited, p. 453-467). Carvone is currently marketed as Talent® in the Netherlands.        In European patent EP 1 051 075 a preparation of a combination of species of the Pink family and species of Alfalfa is described (ComCat®) which reveals within a specific ratio α synergistic bio-stimulatory effect. ComCat® has demonstrated consistent plant growth enhancement and physiological efficiency in the treated plant's utilization of available nutrients. ComCat®, which enhances the health of vegetables, flowers and agricultural crops, is not a fertilizer substitute but, instead, it is a biological enhancer which stimulates the plant to more properly utilize available nutrients. Moreover, it activates and induces allelopathy and disease resistance in the treated plant and stimulates greater production of sugars, which are the building blocks for cellulose and fruiting bodies. The result is a more productive, healthier plant with stronger plant stalks, better flowering and greater fruit biomass (Agraforum Germany, 2002, Technical data sheet).        